But as far as I can tell inspecting the file with a GUI, neither the lines nor the layers have any line weight associated to it. Furthermore, if I simply resave the same file with a different software and "dwg2bmp" it, I get the right rasterization:

In your code, are 0 the vertex index and 7 the line width in drawing units? Should I then iterate over all vertices in the polyline, setting Start and EndWidth? How about using RPolyline.setWidth(double) to set the width of the whole thing?